1. Field of the Invention
The invention relates to a semiconductor wafer composed of monocrystalline silicon which has good gettering properties and is substantially free from defects in a region relevant to the integration of electronic devices. The invention also relates to an economical method for producing such semiconductor wafers.
2. Background Art
Two central requirements are made of a high-performance semiconductor wafer composed of monocrystalline silicon with regard to the defect distribution. Thus, the region of the semiconductor wafer into which the electronic devices are integrated must be as free as possible from defect types having a size in the range of the size of the structure widths of the electronic components or larger. These defect types include, in particular, COPs (“crystal originated particles”), provided that they attain the corresponding size, BMDs (“bulk micro defects”), OSFs (oxidation induced stacking faults) and OSF nuclei (nuclei for oxidation induced stacking faults). COPs are accumulations of vacancies that form voids in the nanometer range. COPs having a disturbing size can be detected for example as “Direct Surface Oxide Defects” (DSOD) with the aid of a preparation method described in U.S. Pat. No. 5,980,720, or else by means of laser scattered light topography, the results of which correlate well with those of DSOD detection. In particular, a laser scanner of the type MO-6 from the manufacturer Mitsui Mining and Smelting, Japan is suitable for automatically detecting COPs having a diameter of approximately 40 nm or larger (COP test). Furthermore, larger COPs can also be detected as “flow pattern defects” (FPD) by means of Secco etch and optical inspection. BMDs are precipitates of supersaturated oxygen which grow from smaller nuclei as a result of a thermal treatment (“anneal”) of the semiconductor wafer. The diameter of the nuclei is typically approximately 1 nm. BMDs are detected for example by IR-LST (“infrared laser scattering tomography”), with confocal imaging in the infrared by SIRM (“scanning infrared spectroscopy”) or, after enlargement (defect etching) for example with the aid of a chromium-containing etching solution, by optical microscopy. BMDs must have an average diameter of approximately 70 nm in order to be able to be detected with SIRM or by defect etching and subsequent microscopy. BMDs starting from a size of approximately 20 nm can be detected directly by IR-LST, for example with the aid of a BMD counter of the type MO-4 from Mitsui Mining and Smelting Co. Ltd., Japan. For detection by means of SIRM or by means of defect etching in combination with microscopy, the majority of the BMD size distribution can be raised above the detection limit by allowing the BMD nuclei to grow at a temperature of typically 1000° C. for 16 hours (BMD-Anneal).
OSFs are silicon stacking faults that can arise if relatively large oxygen precipitates acting as nuclei (OSF nuclei), the diameter of which typically lies within the range of 10 to 35 nm, grow as a result of oxygen accumulation after a surface oxidation in a moist or dry atmosphere at temperatures of around approximately 1050-1100° C. for a number of hours. OSF nuclei arise during the cooling of a silicon single crystal pulled from a melt, in regions which have an excess of vacancies despite COP formation. The formation of OSF nuclei can be intensified by the presence of nitrogen and by slower cooling of the single crystal.
The region of the semiconductor wafer that is relevant to electronic components should contain a minimum of OSF nuclei since OSFs are responsible for gate oxide breakdowns. Furthermore, even OSF nuclei can cause thin gate oxides having a thickness of not more than 7 nm to undergo breakdown. The breakdown strength of test capacitors in a GOI (“gate oxide integrity”) test is therefore a reliable indicator of the detection of OSFs and OSF nuclei.
Outside the region of the semiconductor wafer that is relevant to components, that is to say in the bulk of the semiconductor wafer, the requirement is for a maximum density of BMDs. They form anchoring sites, so-called intrinsic getters, for metallic impurities and in this way reduce the concentration thereof in the region relevant to components. The size of the BMDs used as getters must not fall below a critical size because small BMDs are not thermally stable and are eliminated in the course of thermal treatments of the semiconductor wafer that are performed during the production of electronic components. A minimum size and minimum density of said BMDs are required in order to ensure a sufficient getter capacity, in particular for nickel and copper (K. Sueoka et al., The Electrochem. Soc. PV 2000-17, (2000), p. 164).
EP1170404 A1 describes a semiconductor wafer composed of monocrystalline silicon and a method for producing it. The semiconductor wafer has a high and radially rather homogeneous BMD density in the bulk and therefore good gettering properties. The latter are essentially ensured by two measures. Firstly, the single crystal that yields the semiconductor wafer is doped with nitrogen since the presence of nitrogen promotes oxygen precipitation and the formation of thermally stable oxygen precipitates. Secondly, use is made of the fact that a comparatively high number of free vacancies are available in the so-called Pv region, which is referred to as the NV region in EP1170404 A1. Like nitrogen, free vacancies are involved in the formation of oxygen precipitates.
It is generally recognized that the ratio of pulling rate V and axial temperature gradient G at the phase boundary V/G during the process of pulling a silicon single crystal, is the essential parameter which can be used to control the formation of intrinsic point defects, that is to say vacancies and silicon interstitials, and the agglomerations thereof. The greater V/G is in comparison with a critical value, the more pronounced is the excess of vacancies that pass into the growing single crystal. The smaller V/G is in comparison with the critical value, the more pronounced is the excess of silicon interstitials which pass into the growing single crystal. If the point defects attain supersaturation during the cooling of the single crystal, they form agglomerates which, in the case of the vacancies, are the cause of the production of COPs. The Pv region arises if V/G is controlled during the process of pulling the single crystal in such a way that the quotient is greater than the critical value and less than a V/G for which a region forms in which OSF nuclei preferably arise (OSF region). There is a high density of agglomerates of vacancies in the Pv region. However, the average diameter of these agglomerates is so small that they are no longer detected by a COP test. However, they can be detected after decoration with copper (decoration test) or with the aid of IR-LST. A suitable preparation method for detection by means of IR-LST is described for example in G. Kissinger, G. Morgenstern, H. Richter, J. Vanhellemont, D. Gräf, U. Lambert, W. v. Ammon, P. Wagner, The Electrochem. Society: Proceedings Vol. 97-22, 32-39 (1997) page 117. The decoration test can be performed for example as follows: On the rear side of the semiconductor wafer to be examined, or of a piece thereof, copper is deposited electrolytically from an HF-containing (10 ml/l HF) aqueous CuSO4 solution (20 g/l CuSO4) (G. Kissinger, G. Morgenstern, H. Richter, J. Mater. Res., Vol. 8, No. 8 (1993), p. 1900). Afterward, the sample is subjected to thermal treatment at a temperature within the range of 900-1000° C. for 5-20 minutes, and is finally subjected to a lustre etch (HNO3: HF=5:1) for 10 to 30 minutes. This is followed finally by a treatment with a Secco etching solution for 30 minutes, after which the defects become visible.
The range of permissible pulling rates, knowledge of which is necessary for controlling the quotient V/G in an intended manner, is usually defined by means of one or more preliminary experiments in which a single crystal is pulled at a varying pulling rate and is subsequently analyzed. For this purpose, the single crystal is cut open lengthwise and the cut surfaces are examined with regard to point defects and the accumulations thereof.
According to the description in EP1170404 A1, even those semiconductor wafers which consist entirely of an OSF region, or those which have an OSF ring surrounded by Pv region on the inside and outside, have good gettering properties. No account is taken of the fact that measures for avoiding harmful OSF nuclei have to be taken as soon as the OSF region is included, and that difficulties as a result of COP formation are virtually unavoidable as soon as a vacancy-rich region is present inside an OSF ring.
A further requirement imposed on the manufacturers of high-performance semiconductor wafers composed of monocrystalline silicon is the ability to economically produce these semiconductor wafers in the highest possible numbers. It is urgently necessary, therefore, for the production method to provide a high yield. However, many difficulties have to be overcome in this regard. One problem, which is also addressed by the present invention, is that the concentration of nitrogen in the single crystal rises owing to the small segregation coefficient of nitrogen depending on the length of the single crystal. In the case of semiconductor wafers which originate from the end portion of the single crystal processed to form semiconductor wafers, the comparatively high concentration of nitrogen can have the effect that they have an excessively high number of OSF nuclei since nitrogen promotes the oxygen precipitation necessary for the nucleation. However, the number of OSF nuclei is not only dependent on the concentration of nitrogen, since semiconductor wafers which originate from the front part of the single crystal section processed to form semiconductor wafers can also have an excessively high number of OSF nuclei.